Flattened ultra-microcellular structure and method for making same



y 1968 R. G. PARRISH 3,384,531

FLATTENED ULTRA-MICROCELLULAR STRUCTURE AND METHOD FOR MAKING SAME FiledNov. 27, 1963 Fl G. ZA

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INVENTOR ROBERT GUY PARRISH BY mam ATTORNEY United States Patent 03,384,531 FLATTENED ULTRA-MICROCELLULAR STRUC- TURE AND METHOD FORMAKING SAME Robert Guy Parrish, Wilmington, Del., assignor to E. l.

du Pont de Nemours and Company, Wilmington, DeL, a corporation ofDelaware Continuation-impart of application Ser. No. 157,820,

Dec. 7, 1961. This application Nov. 27, 1963, Ser.

19 Claims. (Cl. 161-159) ABSTRACT 0F THE DISCLOSURE Paper andsemi-textile-like sheets of a crystalline hydrocarbon polymer composedof flattened polyhedral shaped cells possessing uniplanar orientationand being substantially aligned within the plane of the sheet. Processfor the preparation of the above sheets by permanently compressing orstretching a crystalline ultramicrocellular sheet material composed ofthe hydrocarbon polymer (for example polyethylene and polypropylene).

This a continuation-in-part of application Serial No. 157,820, filedDecember 7, 1961, and now abandoned.

This invention relates to a synthetic paper material and relatedproducts. More specifically it relates to paper-like andsemi-textile-like structures prepared from ultramicrocellular,crystalline polyhydrocarbon sheet material.

A major deficiency of cellulosic paper products is their watersensitivity, as evidenced by their poor wet strength and dimensionalinstability with changing humidity. Certain special treatments, such asresin impregnation or wax coating, have been used to mask thisdeficiency and permit construction of useable bags, towels, cartons,etc. In contrast, the polyhydrocarbon materials of this invention arenotable for their water insensitivity, and thus sheet products of thesematerials do not require any special water proofing treatments.Unfortunately, this very feature generally leads to other problems whenit is desired to print on polyhydrocarbon objects (e.'g., films), andcurrent practice is to subject these materials to a special treatment(for example, apply an ink receptive coating, buff or toughen thesurface, or alter the surface through chemical reaction or high energyirradiation) to render the surface ink receptive. It is a specialfeature of the polyhydrocarbon sheets of this invention that theyexhibit excellent ink receptivity directly as produced.

The products of the invention greatly excel corresponding cellulosicproducts in such properties as tensile and tear strength, toughness, andopacity at equal basic weight (or equal opacity at A to A the basicweight). The lower densities of these products as compared withcellulosic papers result not only in a competitive raw material cost peritem, but also economy on shipping charges per item as well. A furtheradvantage of these polyhydrocarbon synthetic papers is their excellentdielectric properties which suits them uniquely for use in electrostaticphotocopying processes as well as for insulating layers in electricaldevices such as capacitors and the like.

It is an object of this invention to provide a polyhydrocarbon sheetmaterial useful as a cellulosic paper substitute having high tensilestrength, toughness, opacity, tear strength and low density and Watersensitvity. It is a further object to provide such materials with inkreceptive directly printable surfaces. Another object is to providesynthetic paper sheet materials with excellent dielectric properties byvirtue of their polyhydrocarbon composition. Still another object is toprovide semi-textile sheet materials. Other objects will appearhereinafter.

In accordance with this invention these objects are accomplished byefiecting a substantial reduction in the ice thickness of a particularcellular sheet material, specifically a low density ultramicrocellularsheet structure comprising a crystalline polyhydrocarbon. By virtue ofthe reduction in thickness, a low density cellular material is convertedinto a paper-like or semi-textile-like product of significantly improvedproperties of tensile strength, toughness and, frequently, tear strengthas Well. Microscopic examination of the product reveals that thecellular nature of the original sheet structure is preserved essentiallyintact although individual cells will be observed as having beencollapsed, flattened or otherwise distorted in shape. Techniques foreffecting the reduction in thickness of the cellular sheet materialinclude the use of mechanical compressive forces applied in a directionnormal with respect to the plane of the sheet and the use of bilateraltension forces applied in a direction across the plane of the sheet.

The preparation of ultramicrocellular structures is described incopending US. application Serial No. 170,187, filed January 31, 1962,now US. Patent No. 3,227,664. The present invention utilizes as astarting material those ultramicrocellular structures which are extrudedin the form of a low density sheet material comprising a crystallinepolyhydrocarbon. As described in the aforementioned U.Sv application,the ultramicrocellular structures are particularly unique owing to thepolyhedral shaped structure of their cells, to the film-like characterof the cell walls, and to the uniform texture and high degree ofmolecular orientation, i.e., uniplanar orientation, existing in thosewalls. Together these features serve to define a class of materialswhich, in comparison with prior art cellular structures, exhibitoutstanding strength and resiliency properties, although fabricated atextremely low densities.

The ultramicrocellular structures to be employed as starting materialsin accordance with the present invention can also be prepared by amodification of the extrusion procedures disclosed in Belgian Patent568,524. The polyhydrocarbon-activating liquid solution is prepared at aconcentration and temperature suitable for producing a cellularstructure, for example as represented by a point in area B of FIGURE 10or 11 of the Belgian patent. This solution is extruded through a slot orannular orifice from its high pressure region into a region of lowerpressure in such a manner that at least 10 bubble nuclei/ cc. exist inthe solution at the instant of extrusion. This may be accomplished inseveral ways, for example by dispersing a quantity of a porous solidbubble nucleation assisting material in the solution or by providing anadequate pressure drop across the extrusion orifice.

The required pressure drop may be estimated from equations given in anarticle by L. Bernath, Industrial Engineering Chemistry, 44, p. 1310(1952). The large volume increase just outside the extrusion slot orannulus, caused by the rapid generation of activating liquid vapor,generally leads to an expansion of the celluar sheet which cannot begeometrically accommodated without the formation of longitudinal folds.These may be minimized by employing Well known blown film techniqueswith the annular die, or other techniques known in the art.

A detailed description will now be given of characteristics of theas-extruded ultramicrocellular structures which, in accordance with thepresent invention, are employed as starting materials for the productionof paper-like and semi-textile-like products. To a large extent it isthese characteristics which give rise to sheet products of unusualproperties following compression or stretching to effect a reduction inthickness.

As regards the as-extruded ultramicrocellular structures, substantiallyall of the polymer is present as filmy elements whose thickness is lessthan 2 microns, preferably under 0.5 micron. The term drained foam isaptly descriptive of such ultramicrocellular structures. The

thickness of a cell wall, bounded by intersections with other walls,does not ordinarily vary by more than 130%. Adjacent walls frequentlywill have generally equal thickness values, such as within a factor of3. The polymer in the cell walls exhibit uniform texture and uniplanarorientation. The apparent density of the ultramicrocellular products isbetween 0.5 and 0.005 g./cc. The number of cells per cc. is desirably atleast although values of 10 or greater are preferred. In general, theultramicrocellular structures may be formed with densities in the rangeof 0.5 to 0.005 g./cc. As a suitable starting material for purposes ofthe present invention, however, they should have a density less thanabout 0.17 g./cc.

In the case of an as-extruded microcellular structure, the cell wallthickness can be determined by microscopic examination of crosssections. Thus 20-60 micron thick sections may be cut from a frozensample with a razor blade. Large cell 50 microns) samples are frozendirectly in liquid nitrogen. Smaller celled samples are preferablyimbedded in water containing a detergent, and then frozen and sectioned.The transverse dimension of one or more cells can also be readilymeasured by freezing and sectioning techniques. The cells are found toexhibit a general polyhedral shape, similar to the shape of the internalbubbles in a foam of soap suds. The average transverse dimension of thecells should be less than 1000 microns, preferably less than 300microns, and the mutually perpendicular transverse dimensions of asingle cell should not vary by more than a factor of three. The ratio ofthe cell volume to the cube of the wall thickness can be calculated andexceeds about 200. For very thin walled samples 1 micron), the wallthickness is preferably measured withan interferometer microscope. Alayer of the sample is peeled off by contact with Scotch Tape. The layeris freed from the tape by immersion in chloroform and subsequentlyplaced on the stage of the microscope for measurement.

The term uniplanar orientation employed with respect to the as-extrudedultramicrocellular structures may be fully understood from the followingdiscussion. As will be described in greater detail in subsequentportions of the specification, the paper-like and semi-textile-likeproducts of the present invention also possess uniplanar orientation.Axial, planar, and uniplanar indicate different types of molecularorientation of high polymeric materials. Axial orientation refers to theperfection with which the molecular chains in a sample are aligned withrespect to a given direction, or axis, in the sample. For example, priorart filaments which have been drawn in one direction only generallyexhibit an appreciable degree of axial orientation along the stretchdirection. Planar orientation refers to the perfection-with which themolecular chains are oriented parallel to a surface of the sample.Uniplanar orientation is a higher type of polymer orientation in that itrefers to the perfection with which some specific crystalline plane(which must include the molecular chain) in each polymer crysallite isaligned parallel to the surface of the sample. Obviously, onlycrystalline polymers can exhibit uniplanar orientation. These threetypes of molecular orientation may occur singly or in combination; forexample, a sample might simultaneously exhibit uniplanar and axialorientation.

Electron difiraction furnishes a convenient technique for observing thepresence of uniplanar orientation. A single cell wall is placedperpendicular to the electron beam. Since the Bragg angle for electrondiffraction is so small, only crystalline planes essentially parallel tothe beam (perpendicular to the Wall surface) will exhibit diffraction.If the sample does in fact have perfect uniplanar orientation, there issome crystallographic plane which occurs only parallel to the filmsurface and, therefore, will be unable to contribute to the diffractionpattern. Thus, the observed pattern will lack at least one of theequatorial diffractions normally observed for an ax ially orientedsample of the same polymer. If the degree of uniplanar orientation issomewhat less than perfect, there may be a few crystallites tilted farenough to contribute some intensity to the diffraction pattern, but atleast one of the equatorial diffraction intensities will be appreciablyless than normal. Thus, for the purpose of this invention, a sample isconsidered to have uniplanar orientation when at least one of theequatorial diffractions appears with less than one-half its normalrelative intensity as determined on a standard which is a randomlyoriented sample of the same polymer.

An alternative and occasionally more convenient technique for detectingthe presence of uniplanar orientation in a sample is to observe theelectron diffraction pattern as the plane of the sample is tilted withrespect to the electron beam. (In case the sample also exhibits axialorientation, the tilt axis is preferably parallel to the orientationaxis.) For uniplanar-oriented samples, first one crystallographicdiffraction plane and then another will assume the position required forBragg diffraction, so that first one and then another lateraldiffraction will appear and then disappear as the sample rotationcontinucs. The more perfect the degree of uniplanar orientation, themore sharply defined is the angle at which any particular diffractionappears. When a plot of dilfraction intensity (corrected for samplethickness variation) vs. angle of sample tilt is prepared for any of thelateral dilfractions, the distance in degrees tilt between points ofhalf-maximum intensity may be readily determined. ()nly samples havinguniplanar orientation will have half-maximum intensity points separatedby or less, and this will serve as an alternate criterion for thepresence of uniplanar orientation.

One precaution must be observed in making this measurement. If thesample field examined by the electron beam is stopped down so far thatit sees only one crystallite at a time, it will always be possible, evenfor a randomly oriented sample, to find some crystallite orientedparallel to the sample surface which would, of course, give an uniplanarorientation diffraction pattern. In order to insure that the uniplanarorientation pertains to the whole film element and not just to onecrystallite, the measurement should be made examining a field of atleast square microns area, which is large enough to include thecontributions from many crystallites simultaneously. Other techniques ofmeasuring uniplanar orientation and their co-relation with electrondiffraction measurements are described in the J. Pol. Sci, 31, 335(1958), in an article by R. S. Stein.

The term uniform texture applied to the polymer in the cell walls of anas-extruded ultramicrocellular means that the orientation, density, andthickness of the polymer is substantially uniform over the whole area ofa cell wall, examined with a resolution of approximately /2 micron. Thisis best determined by observing the optical birefringence in the planeof a Wall of a cell removed from the sample. For ultramicrocellularsamples with a net over-all axial orientation, the individual cell wallswill normally exhibit an axial orientation in addition to the requireduniplanar orientation. In the birefringence test, such products of thepresent invention will show a uniform extinction over the whole area ofthe cell wall. Samples with no net axial orientation must show a uniformlack of birefringence over their whole area rather than numerous smallpatches of orientation with each patch oriented at random with respectto the others. Lacy or cobweb-like cell walls, of course, do not haveuniform birefringence over the whole area of a cell wall, and suchproducts are readily distinguished from the uniform textured products ofthis invention. After mechanically collapsing an asextrudedultramicrocellular sheet in accordance with the invention, the cellWalls may no longer exhibit uniform texture.

The present invention utilizes as a starting material an as-extrudedultramicrocellular sheet structure which has been prepared from apolyhydrocarbon, e.g., a high molecular weight hydrocarbon polymer.Since only crystallizable or crystalline polymers are suitable in theprocess of preparing the ultramicrocellular sheets, for the purposes ofthis invention linear polyethylene, polypropylene and crystallizablecopolymers or graft polymers of ethylene and propylene with othermonomers such as l-olefins of up to ten carbon atoms are preferred.Blends of such polymers are also suitable. Other operable polymersinclude poly(3-methy1 butene), poly(4-methyl pentene), isotacticpolystyrene, and the like. Suitable activating liquids are described inthe aforementioned U .S. application 170,187 and in Belgian Patent568,524. Among those most preferred are hexane, pentane, butane,methylene chloride, and trichlorofluoromethane. In order to take fulladvantage of the process and produce products with the highest tensileproperties, it is preferred to use polymers of low melt index.

The as-extruded ultramicrocellular sheet structures will consistessentially of polyhedral shaped cells including both closed cells andopen cells (tubular structures of varying length arranged in asponge-like structure) in any proportion, depending on the choice ofoperating conditions. There may also occur a minor amount of fibrillarmaterial,

but the as-extruded ultramicrocellular sheet is a coherent unitarystructure.

According to the present invention, the low density asextruded sheetsare mechanically treated to permanently reduce their thickness and toincrease their density. It is a surprising feature of this inventionthat such mechanical creases the tensile strength of the sheets. Atensile above 5 I lb./in.//oz./yd. in the machine direction and above1.5 lb./in.//oz./yd. in the transverse or cross direction is readilyattained. The work-to-break value in the machine direction which is ameasure of the toughness of the sheets, determined by the area under thestress-strain curve is markedly improved. In general, the tear strengthis also improved in the machine direction.

The collapsed, densi-fied sheet prepared as described hereinafter, maybe described as an integral structure of a crystalline hydrocarbonpolymer comprising flattened polyhedral cells whose walls have anaverage film thickness below 2 microns and are aligned substantially inthe plane of the sheet. The walls of the individual cells possessuniplanar orientation and the smallest cellular dimension, which wouldcorrespond to the height of a flattened cell is below 50 microns,preferably below microns. The second largest dimension of the polyhedralcells is on the average between 1 and 3000 microns, but at least 3 timesthe height.

The measurement of the wall film thickness may be made with aninterferometer microscope at 400 magnifications. In one technique alayer of the sheet is peeled oif by contact with Scotch Tape. The layeris freed from the tape by immersion in chloroform and placed on thestage of the microscope for measurement. The second largest cellulardimension is measured on a polarizing microscope at 300x, the dimensionbeing the distance between the ridges representing side walls orremnants thereof on a layer of the sheet obtained as above. The sampleis immersed in oil of about 1.5 refractive index for this determination.

The height of the flattened cell is calculated by the formula where d isthe density of the sheet in grams/ cc. determined film thicknessdetermined as described above. The fact that the cellular walls aresubstantially aligned in the plane of the sheet is evidenced by thepositive birefringence in a plane perpendicular to the surface of thesample, i.e., the index of refraction measured parallel to the surfaceof the sheet is greater than the index of refraction measuredperpendicular to the surface of the sheet. This determination is made inthe manner described by Chamot and Mason Handbook of ChemicalMicroscopy, vol. 1. A 10 micron section of the sheet in ice is cut asthe specimen with a freezing microtome. The sign of the birefringence ismeasured with a low power polarizing microscope and a first order redplate. The magnification employed should preferably be too low todistinguish the individual cell walls, and the positive birefringencemust apply to the section as a whole, not just the surface layers of thesample. These 10 micron sections may often conveniently be used for theinterferometric determination of wall thickness, provided the walls areplaced parallel to the plane of the sample by compressing the section.

Two somewhat difierent mechanical treatments are particularly suitablefor converting the as-extruded ultramicrocellular sheets to thecollapsed products of this invention. One such treatment involvesapplying mechanical forces in a direction normal to the plane or facesof the sheets. The sheets are thus mechanically compressed beyond theiryield point with pcrmant reductions in thickness ranging from factor of/3 to or less. The yield point is achieved when the thickness reductionis of sufficient magnitude that full recovery of the original thicknessis not experienced upon release of the mechanical forces. If required,any longitudinal folds in the extruded sheets may be removed prior topressing by mechanically spreading or stretching th esheet transversely.The pressing may be performed continuously, as by calender rolls or twojuxtaposed moving belts, or discontinuously as by any type of hydraulicpress. Although the sheets are compressed under sufficient pressure thatthey permanently remain deformed, the pressure must not be so high as tofuse the cellular structure into a film like product with total loss ofthe cellular character and unique polymer orientation of the startingsheet. By stopping just short of this point it is possible to prepare atranslucent or transparent glassine-like product with superior tensileproperties, water vapor impermeability and grease resistance. Pressuresfrom about 10 to 10,000 p.s.i. or greater are ordinarily employed. Areasonable pressure for preparing the paper-like products of thisinvention has been determined to be 500 p.s.i., although both higher andlower pressures are useable. The semi-textile-like materials areordinarily prepared at lower pressures, such as 10 p.s.i. The durationof application of pressure is not particularly critical, and may rangefrom a fraction of a second to several minutes or more. The pressingtemperature may range from room temperature or lower up to thecrystalline melting point of the polymer. Naturally, the variables ofmaximum pressure, pressing temperature and duration of pressureapplication are all inter-related, and any one may be adjusted towardsome desired value by suitable change of the other two. It is, ofcourse, quite feasible to impart textured patterns or designs to theseproducts by using embosing rolls or platens for pressing poeration.

The use of calender rolls to mechanically compress an as-extrudedultramicrocellular sheet structure is schematically illustrated inFIGURE 1. As shown therein an autoclave or other suitable vessel 15equipped with agitator 1, discharge valve 16, and slot shaped extrusionorifice 2 is charged with a crystalline hydrocarbon polymer andactivating liquid and brought up to the required temperature andpressure. Valve 16 is then opened and extrudate 6 in sheet form (sideview or thickness shown) is discharged from the orifice. The thicknessdimension of the sheet quickly increases many fold as the activatingliquid vaporizes. The as-extruded cellular sheet is then led to anysuitable means which will compress the sheet beyond its yield point. Apair of heated calender rolls 23 and 24 is illustrated in the drawingfor achieving this objective. After permanent reduction in thickness,the sheet is collected on windup roll 27.

A second mechanical treatment for achieving a permanent thicknessreduction in the as-extruded cellular sheets comprises bilaterallystretching the sheets, as in a drawing operation. The extent andduration of this stretching must be chosen to produce a permanentreduction in thickness by at least a factor of three. A draw ratio of 2Xor more will frequently suffice. The stretching may be carried outcontinuously (e.g., on the run) in either the machine direction,transverse direction or a combination of both, or the stretchingoperation may be performed on discrete pieces of the as-extrudedcellular sheet. Such lateral stretching operations may actually elongatethe individual cell Walls, so that whereas the transverse dimensions ofthe cells of the as-extruded samples may range up to 1000 microns, thecells of the stretch-collapsed samples may range up to 3000 microns inwidth. As is the case with mechanical compressing technique, the sheettemperature may range from room temperature or lower up to thecrystalline melting point of the polymer.

A pref rred variant of this technique consists of stretching the sheetsto at least about 150% of their initial larger dimensions in twomutually perpendicular directions. This modest degree of stretching doesnot appreciably further draw and orient the polymer in the filmy cellwalls, but serves simply to collapse the structure. It may be visualizedas merely increasing the distance between the opposite edges of thepolyhedral cells to cause internal adjacent faces to come into contactby collapse of the structure. Products prepared in this Way thus haveboth width and length increased to at least about 150% of that of theoriginal as-extruded sheet, whereas the area of the sheets compresesedby applying force normal to their surface remains unchanged. Althoughproducts collapsed in either way have their film-like cell wallssubstantially aligned in the plane of the sheet, as described earlier,there are certain differences between the two. In the compressedproducts, for example, the cell walls originally aligned approximatelyperpendicular to the sample surface are crumpled and folded during thecollapsing process, as indicated scematically in FIGURE 28, whilecorresponding walls in the stretched products have been tilted aroundinto the plane of the sheet, and remain extended as indicatedschematically in FIGURE 20. This difference in structure gives rise tothe following features, making the stretched samples preferred forcertain uses: the stretched products have a higher luster, higherinitial modulus and tensile strength and a higher tensile yieldstrength. Furthermore, although the stretched and the compressed sheetproducts are both opaque (due to multiple reflection and scattering oflight at the interfaces of their residual cellular structure), both canbe made transparent by applying additional pressure sufiicient to fullycollapse their cellular structure and bring the cell walls into intimatecontact. However, the stretched sheets are much more readilypressure-clarified than the compressed sheets.

It has been discovered that the tensile properties of these products arestrongly directional dependent. For example, the tensile strength in themachine direction may be five times that in the transverse direction. Itis thought that this feature is related to weak longitudinal channels inthe cellular sheets produced during the extrusion operation.

Products with balanced properties may readily be produced by crosslapping and laminating the single sheet products. Ordinarily, in atwo-ply cross-lap, the two sheets will be placed so that the machinedirections are at right angles, or at an angle of at least 30. It ismost surprising to observe that such cross-lapped sheets exhibittremendous increases in tear strength, sometimes fifty times that of acomparable weight cellulosic product. It is postulated that thisstriking increase in tear strength may depend on the presence of therelatively weak longitudinal channels in the sheets. Thus in across-lapped structure, a tear cannot be made to propagate very far inany direction without crossing one of the weak channels, and each timethis occurs, the tear must effectively be re-initiated, which for suchmaterials requires a higher force than that required for simple tearpropagation.

Suitable bonding between the cross-lapped sheets may be achieved eitherby application of adhesive or by selfbonding. Useful adhesives may be ofseveral types such as pressure sensitive adhesives, e.g., Formica glue,and melt adhesives, e.g., Elvax, or even branched polyethylene.Self-bonding can be achieved without any applied adhesive by pressingthe cross-lapped polyhydrocarbon sheets at temperatures within a fewdegrees of the polymer melting point. The duration of heating should beheld to a minimum to prevent appreciable relaxation of the polymerorientation in the cellular structure proper, since this leads to adegradation in physical properties. If a suflicient degree of bonding isnot attained, these superior tear strengths are not observed, since thecomposite structures will fail by delamination rather than by tearpropagation and reinitiation. In the preparation of crosslappedstructures, it is not necessary to first press the individual sheets. Infact, it is preferred to cross-lap the as-extruded sheets, coated withadhesive if desired, and press and bond them all in the same operation.

The cellular sheet products of this invention may also be subjected to athermal annealing treatment comprising exposing the sample to atemperature between the glass transition temperature and the polymercrystalline melting point. Under. these conditions a change incrystallite size occurs, the c-axis dimension becoming shorter while thetransverse dimensions grow larger. Owing to the initial uniplanarorientation, this crystallite size change during annealing apparentlyrequires that the polymer molecules contract from their original fullyextended configuration (parallel to the cell wall surface) to assume amore normal folded chain configuration. This molecular rearrangementdoes not destroy the perfection of crystallite uniplanar orientation,but does create a tendency for each cell wall to contract in area duringthermal annealing. Evidence tends to indicate that such is attended by aloss in uniformity in thickness of the film-like cell walls, at least ona micro-scale, as the crystallites seem to grow laterallye.g., in thewall-thickness directionat the expense of the quantity of polymer in thesurrounding regions. As a consequence, a sample which is unrestrainedduring the annealing treatment is observed to shrink in size. However,if the cellular sheet is subjected to bilateral restraining forces(e.g., by clamping its edges or by heating between press platens), thetendency of the individual cell walls to contract during annealing setsup internal stresses in the sample. The results appear to be almost asif the sample had been bilaterally drawn: the cells collapse in thethickness-dimension of the sheet and the tensile properties of the sheetincrease. Thus, annealing increases the initial modulus, tensile yieldstrength, and Clark Stiffness (TAPPI Test T-451) of either compressed orstretched sheets, thus further enhancing their utility, for example, asprinting paper where low deformation and high stiffness are desirable.It appears that some rupturing of the cell walls, or perhaps some fusionof the juxtaposed cell wall faces, must also occur, as samples areannealed under restraint.

A special technique exists for producing a collapsed, bonded, balancedproperty cellular sheet in a continuous operation. If the cellular sheetis generated in blown tube form by extrusion through an annular die, theinternal gas pressure between the die and a downstream nip/drawing rollassembly may be adjusted to a sufiiciently high value to produce alateral stretching of the tube by at least 1.5x beyond its normalexpanded diameter. If the speed of the driven nip/drawing roll isadjusted to provide a simultaneous machine-direction drawing of at least1.5x,

the conditions for mutually perpendicular bilateral stretch ing are met,and a collapsed cellular product is produced directly. Furthermore, thisproduct is observed to be internally bonded, even though no externalsource of heat is applied. It appears that by collapsing the product inits freshly formed condition, sufiicient residual activating liquid ispresent to plasticize the material so that internal bonding is achievedeven at ambient temperatures.

The micro dimensions contribute several important features to theseproducts, namely, the high opacity of the paper-like materials resultingfrom the efiicient light scattering ability of the micro elements, andthe ability to produce uniform sheets at extremely low basis weights,down to 0.1 oz./yd. The discovery that the surfaces of thepolyhydrocarbon sheet products are ink receptive directly as prepared ishighly surprising in view of the hydrophobic nature of the polymer. Theink does not penetrate the sheets to any appreciable extent, nor does itrun or smear into adjacent areas by capillary action. These sheets maybe printed satisfactorily on ordinary letter press and lithographyequipment, or even with commercial electrostatic photocopying equipment.

It is surprising that these cellular sheets consisting of polymericelements which have already been oriented in the extrusion process andpressed can be further oriented by a drawing step. By this technique,paper-like products with tensile strengths as high as 61lb./in.//oz./yd. have been prepared.

As will be illustrated in greater detail in the examples which appearhereinafter, the collapsed sheet products of the invention have utilityin a great variety of applications, whether they be used alone or incombination with other materials. The as-extruded or uncollapsed sheetsalso, of course, have many utilities directly as prepared. Particularlyuseful are those samples having a high percentage of closed cells whichimpart a pneumatic character to the sheets. Although the as-extrudedsheets are ordinarily below A" in thickness, they may be laminatedtogether to produce thicker structures, which may be made more or lessflexible depending on whether the bonding is accomplished in discreteareas only, or over the full areas in contact. For certain application,e.g., in semi-textile products where higher breathability is desired, orin upholstery spring insulator pads Where drumming is to be minimized,it is desirable to perforate these ultramicrocell ular sheets withholes, either by cutting, melting, punching, needling, or othertechniques known to those skilled in the art. When even lower densityresilient materials are desired, these pneumatic cellular sheets may belanced with slits and laterally extended to open up the apertures sothat a given weight of cellular structure occupies an even larger bulkvolume. In any event, the collapsed sheet products of the presentinvention frequently constitute an excellent surfacing layer whenlaminated to an as-extruded ultramicrocellular sheet for the foregoingapplications.

This invention is further illustrated by the following examples:

EXAMPLE I A mixture of 1,000 grams of linear polyethylene (melt indexequal 0.5), 750 cc. methylene chloride activating liquid, 135 gramschlorodifiuoromethane, and 5 grams Santocel (Monsanto trademark forsilica aerogel) as a nucleating agent was charged into a 3 literstainless-steel pressure vessel. The contents were heated and mixed 6hours at 150 C. to form a homogeneous solution. Prior to extrusion, thepressure vessel was connected to a source of nitrogen gas at a pressureof 450 p.s.i. Extiusion occurred through a 10 mil annular orifice 3inches in diameter with a 0.0625 long parallel land at a velocity ofapproximately 500 y.p.m. The microcellular sheet product thus producedis in tubular form, approximately 10 inches in diameter, and exhibitslongitudinal corrugation, presumably formed during the lateral expansionfrom the extrusion die. The optical thickness of the cell walls variesfrom 0.3 to 0.6 micron from the interior to the surface of the sheet.The thickness of polymer at intersections of cell walls is less than 1micron, characteristic of a polyhedral walled multice'llular structure.The cell size ranges from to 200 microns. The polymer mole cules in thecell walls are oriented parallel to the plane of the wall to within 10degrees indicating a high degree of planar orientation. Electrondiffraction indicates the 200 reflection is completely absent in most ofthe cell walls studied, indicating a high degree of uniplanarorientation. The polymer in the line of intersection of bubble wallsshows a high degree of axial orientation along the direction of theintersection. The product prepared according to this example has a basisweight of 0.4 oz./yd.

These microcellular sheets have a bulk density of 0.02 gram/cc. as spun.By applying pressure up to 500 p.s.i. to the face of these sheets for 1/2 minutes at a temperature of 50 C. a range of sheet products ofincreased density up to 0.5 gram/cc. is obtained. These products haveremarkably high strength in the machine direction of 22lbs./in.//oz./yd. The tensile strength in the transverse direction isonly 4 lbs./in.// oz./yd. When two such sheets are cross-lapped andbonded with adhesive (or self-bonded by pressing at temperatures nearthe polymer melting point), they form a composite with exceedingly hightear strength of 2 lbs.//oz./yd. (about 20 to 50 times higher thanordinary cellulosic sheets) and tensile strengths as high as The lightlypressed cross-lapped sheet structures are leather-like in theirsuppl-eness, hand, and bending characteristics. They have been made intoobjects such as gloves, carrying cases, insulating bags, slippers andthe like. It is possible to prepare such products exhibiting a range ofwater vapor transmission as desired in the range from about 1 to 60grams/meter /24 hours by varying the proportion of open and closed cellsby extruding closer to, or farther from the fibrillation line, shown asline (1) in FIGURES 10 and 11 of the Belgian patent.

The cross-lapped sheet products pressed at the higher pressures arepaper-like, being light-weight, thin, flexible, and opaque. Theseproperties plus their excellent tensile strength, tear resistance, andwater insensitivity ideally suit these sheets for use as a premiumbagging material. Another very interesting product usefiul as acarbonless copy paper or thermographic copy paper is produced when acolored adhesive is employed in the laminating step, or when a pressedmicrocellular sheet is laminated to one or both sides of a coloredsheet. The color is ordinarily obscured by the opacity of themicrocellular sheet. However, the color may be revealed in selectedareas to form characters or patterns by selectively coalescing theoverlying microcellular regions to destroy their opacity by applicationof heat or pressure. The heat generated in a thermographic copyingprocess is adequate to effect such selected area clarification, andcopies of suitable originals are readily prepared in commercial oflicethermographic equipment. Pressure coalescence and clarification ofselected microcellular areas may be accomplished with a stylus or othermeans, as for example, the force exerted by the type face of atypewriter key. As many as eight clear legible copies of good contrasthave been prepared simultaneously in a single typing on a stack of eightsuch laminated sheets. Ordinarily, a white opaque microcellular sheetwith a dark colored adhesive, as may be prepared by mixing a pigment ora dye with any suitable adhesive, is preferred for such carbonless copypapers. However, other combinations are possible, for example, bypreparing a colored microcellular sheet from a pigmented or dyecontaining polymer solution.

Incorporation of acicular particles such as potassium titanate willincrease both bending stilfuess and opacity of the microcellularproducts of this invention. For example, a cross lapped pressed sheetproduct is prepared as just described except that K Ti O particlesapproximate-1y half micron in diameter and 50 microns long are chargedinto the pressure vessel with the other ingredients. At a loading of4.4%, based on the weight of the pressed microcellular sheet, an opacityof 92% is achieved at a basis weight of only 0.7 oz./yd. while a controlmicrocellular sheet containing no potassium titanate reaches 92% opacityonly when the basis weight is raised to 1 oz./yd. In further comparisona commercial clay-loaded cellulosic magazine paper achieves 92% opacityonly for a basis weight of 1.8 oz./yd. (Another microcellular sheetcontaining 5.8% of smaller potassium titanate particles exhibits anopacity of 92% at a basis weight of 0.4 oz./yd. For 1 z./yd. basisweight sheets, the potassium titanate reinforced sample has a Clarkstiffness ap proximately twice that of the control. The stiifness ofboth these sheets increases by an additional factor of 2X upon beingannealed at 126 C.

A microcellular sheet prepared according to the recipe of Example I at abasis weight of 0.3 oz./yd. was pressed at 500 p.s.i. and cut intostrips by 3" to serve as a backing material for pressure sensitivebandages. Several /2" slits were cut in the longitudinal direction inthe center of the strips to impart breathability. This central area wasthen covered with a cheese cloth simulation of the gauze pads employedin commercial bandages. Two A" strips from the same pressed sheetmaterial structure were cross lapped and laminated to the 3 strips ateach end of the centrally located pad to provide additional strength andact as tear stoppers. The arms of the strip were coated with pressuresensitive adhesive. When desirable as an aid in application to complexshapes, the arms of the bandage may be torn into longitudinal strips upto the point of the cross lapped tear stopper strips. These bandagespossess a balanced combination of ease of handling, stretching,conformability, ,levelingwith the skin, skin folding characteristics,tension release due to creep, minimum loss of touch sensation, etc.

High strength light-weight ribbons or tapes may be prepared from pressedmicrocellular sheets by laminating strips to a parallel array ofreinforcing cords. One such sample consisted of three layers ofmicrocellular pressed sheets laminated parallel to a grid of nylon tirecords in parallel array at A2" spacing at a total basis weight of 4.6oz./yd. 63% of which was tire cord. This tape had a thickness of 45 milsand a tensile strength of 120 lbs/in, with elongation of 246%.

A similar laminated structure is prepared as follows: A microcellularsheet was prepared according to the recipe of Example I except that thequantities of chlorodifluoromethane and Santocel were 200 grams andgrams respectively. A sample of commercial nylon yarn (42068- 1-Z-300)having a tenacity of 7.9 grams/ denier and 16% elongation was glued tothe surface of this sheet in a parallel array of 10 ends per inch usinga 1% solution of Scotch Tape glue in chloroform as an adhesive. Twosections of this material were cross lapped at 90 with their yarnsurfaces in contact using an additional quantity of the same adhesive,and the composite structure was pressed at 400' p.s.i. and 50 C. for 1/2 minutes. This laminated structure had a total basis weight ofapproximately 2 oz./yd. and contained approximately equal weights of themicrocellular sheet and nylon yarn. This laminated sheet was tested as aprimary backing material for tufted rugs and carpets. Even after mockpunching on a carpet tufting machine, it exhibited excellent propertiesof 97 pounds grab tensile and 23% elongation. These results wereconfirmed by tufting a sample with 501 nylon carpet yarn on commercialequipment, dyeing the sample, etc. Processing was satisfactory and thesample exhibited good properties. The microcellular sheet in this caseserves as a convenient and inexpensive carrier for the non-woven grid ofnylon yarns which give superior strength, dimensional stability,covering power and tuft holding capacity to the carpet backing material.

TABLE Basis Strip Tear Elonga- Sample Weight, Tensile, Propagation,

ozJyd. lb. tion, lb. percent Reinforced 1. 6 16. 2 17 MicrocellularControl 1. 8 26 6. 4 132 lb. kraft 2. 5 11 l 3 2 1 Too low to measure.

Pressed microcellular sheets are also useful as a surfacing material.They may be laminated to almost any substrate including metal, wood,cardboard, and impregnated non-woven fabrics as well as other types offabrics such as knits, felts, Wovens and non-Wovens. A particularlyinteresting example is prepared by laminating four layers ofmicrocellular sheets to one surface of a 16- mil vinyl film substrate.Two of the four layers were made using an annular die with a 10 mil gap,and the outer two layers were made using a 5 mil gap die. The totalbasis weight of the four plies was 2 oz./yd. They were laminated to thevinyl film using Rboplex AC 33 adhesive to produce a smooth surfacedstructure 40 mils thick. This structure bears a remarkable resemblanceto leather in many respects. First, like leather, mechanical working ofthis sheet develops a fine grained leather-like surface texture. The'low heat conductivity of the microcellular surface leads to a neutralwarm hand which is remarkably similar to leather, as is its response tosliding frictional forces, resistance to scuffing, and peeling behavior.These property similarities to leather may well be reflections of thestructural similarities to the non-collapsed cellular surface layer ofleather. The surface grain size referred to above may be controlled byvarying the number and thickness of microcellular sheets, thickness ofsubstrate, pressure during working, etc. In addition, thesethermoplastic surfaces may readily be embossed with sharp clear images.

EXAMPLE II The equipment of Example I was used and 1,000 grams of acopolymer of ethylene and l-octene of density 0.937 (correspondingapproximately to a 96/4 copolymer) and melt index of 0.54, 750 ml. ofmethylene chloride, grams chlorodifluoromethane, and 5 grams Santocelwere charged into the 3 liter pressure vessel. A solution of thecopolymer was formed by heating the mixture to 150 C. for 7 hours. Theautogenous pressure of 385 p.s.i.,g. was increased to 405 p.s.i.g. justprior to extrusion by connecting the pressure vessel to a source ofnitrogen. The microcellular copolymer sheet produced was cross-lappedusing an adhesive (a commercial mixture of a wax plus a vinylacetate/ethylene copolymer, Elvax 250), and pressed two minutes at 70 C.and 500 p.s.i. The resulting sheet of basis weight: 1.3 oz./yd. hadtenacity=9.8 lbs./in.//oz./yd. elongation=%, modulus=42 lbs./in.//oz./yd. and work-to-break=9.1 inch lbs./in. //oz./ yd.

EXAMPLE III The mixture of Example I is modified by adding 61 grams morechlorodifiuoromethane and 5 grams more Santocel. This increases theautogenous pressure of the spinning solution at C. to 495 p.s.i.g. The 3inch annular orifice is changed from a 10 mil gap to a 5 mil gap, andthe solution is pressured with nitrogen to a total of 720 p.s.i.g. justprior to extrusion, which occurs at 160 y.p.m. The microcellular sheetthus produced has a tensile strength of 30 lbs./in.//oz./yd. This issurprisingly high for a sheet Whose basis Weight is only 0.16:0.01oz./yd. since the normalized properties for sheet products generallyfall off rapidly as the basis weight drops below 1 oz./yd.

This sheet is cross-lapped and laminated using Scotch Tape glue, andpressed at 50 C. and 500 p.s.i. to form a composite sheet whose totalbasis weight equals 0.40 oz./yd. tenacity=16.4 lbs./in.//oz./yd.elongation=129%, modulus=30 lbs./in.//oz./yd. work-tobreak=l3 inchlbs./in. //oz./yd. and Elmcndorf tear: 17.7 g.//g./m (Tappi standardspecimen size). These sheets are further remarkable in that thenormalized water vapor transmission is not only the same for the singleand crosslapped sheets, but also equivalent to that for polyethylenefilm. This indicates that even at this very low basis Weight, thesepressed microcellular sheets do not have pin holes or gross defects.This excellent uniformity at such low basis weight could only beattained by microcellular products of the present invention.

EXAMPLE IV To a mixture of 50% linear polyethylene of melt index 0.9 and50% pentane (Phillips Pure Grade) was added 1.5% Santocel (based onpolymer weight). This mixture was confined in a pressure vessel beneatha floating piston above which 900 p.s.i. of nitrogen pressure wasapplied, heated to 155 C., and extruded through a /4 by 0.010 slothaving an axial length of 0.015 inch. The product was a tape about fourinches wide having a basis weight of 0.93 oz./yd. a tensile strength of23.5 lbs./in.//oz./yd. elongation of 110%, modulus of 226lbs./in//oz./yd. and a work-to-break of 16.5 inch lbs./in. //oz./yd.Although this product is substantially monolithic (i.e. not fibrillatedor fractured into gross pieces), the conditions under which it wasprepared are such as to cause rupture of substantial numbers of itsmicrocells in such a way as to form interconnecting tunnels or channelsor intercommunicating cells. This feature leads to a substantiallynon-pneumatic product while still retaining the excellent tensileproperties reported above. The product exhibits a substantial de gree ofuniplanar orientation as indicated by less than half the normal electrondiffraction intensity shown by certain reflections.

EXAMPLE V A mixture of equal parts of linear polyethylene of melt index0.75 and linear polypropylene of melt index 1.08 was prepared by meltblending the components in a screw extruder. 1,000 grams of the polymerblend, 750 cc. methylene chloride, grams Santocel, 170 grams ofchlorodifiuoromethane were charged into a pressure vessel. The mixturewas heated six hours at 150 C. to form a spinning solution whoseautogenous pressure of 450 p.s.i. was increased to 650 p.s.i. withnitrogen pressure just prior to extrusion through a three inch annulardie with a 5 mil gap. The microcellular pneumatic sheet thus preparedexhibited desirable properties characteristic of each component. Forexample, the polyethylene component contributed a degree of drawabilityand fairly good transverse tensile properties to the sheet, while thepolypropylene component contributed greater stiffness and highertemperature resistance than found for similar 100% linear polyethylenemicrocellular sheets. In fact, this product will withstand briefexposure to temperatures as high as 150 C. without melting, whereas themelting point of linear polyethylene is approximately 135 C. There wasno indication of phase separation in the spinning solution, but the twopolymers appear to have frozen out at difierent stages in the spinningoperation, as might reasonably be expected from the 30 difference intheir melting points. Although the microcellular sheet is an integralstructure, it gives the visual impression of being composed of a networkof fine strands of microcellular material dispersed throughout itsvolume and aligned parallel to the machine direction (presumably thepolypropylene component) imbedded in a continuous microcellular matrix(presumably the polyethylene component).

EXAMPLE VI A four-ply composite paper is prepared from sheets extrudedas in Example III, cross lapped and laminated with Carters RubberCement, and pressed at 300 p.s.i. at room temperature for 15 minutes.This 1 oz./yd. paper is annealed in 20 p.s.i. steam to increase itsstilfness by a factor of about two. It is printable employing bothoffset and letter press techniques. The type of ink used is notcritical, but those which dry by oxidation, polymerization or solventevaporation are preferred. The annealing step has no efiect on the inkreceptivity of the surface. The annealed sample cannot be post-inflatedto its original low density.

EXAMPLE VII A linear polyethylene sheet extruded as in Example I isdrawn 2.2x on a hot bar heated to C. The asextruded tensile strength is15 lb./in.//oz./yd. which increases on drawing to 60.5 lb./in.//oz./yd.The modulus shows an increase from 1.3 to 605 lb./in.//oz./yd. Allproperties are measured in the machine direction. The hot drawing stepresults both in collapse of the structure to increase its density and inthe formation of internal bonds.

The cross lapped drawn sheets have a tensile strength of 50lb./in.//oz./yd.

EXAMPLE VIII A 1 oz./yd. paper-like linear polyethylene pressed sheet issubjected to further pressing at 125 C. and 4,000 p.s.i. with anhexagonal faced die. The polyethylene sheet is separated from the lowerpress platen by a hardpressed paper-makers blotter. This operation formsa translucent hexagonal window in the opaque sheet. This sheet is thenformed into a bag, the required seams being formed with a commercialultrasonic sealing machine. The window in the face of the bag permitsviewing its contents without opening the bag. This example dem onstratesan application of this unique product utilizing its excellent tear andtensile strength and optional opacity or translucence.

EXAMPLE IX A polypropylene sheet product is prepared according to themethod of Example I using 1,000 g. of polypropylene of melt index 1.08,750 cc. methylene chloride, 283 g. chlorodifiuoromethane, and 2.0 g. ofSantocel. The autogenous pressure at C. of 520 p.s.i. was increased to530 p.s.i. by nitrogen pressure just prior to extrusion through the0.010" gap 3" annular die.

The following properties were measured on this product, each pair ofnumbers indicating the result as determined in the machine direction andtransverse direction 1 EXAMPLE IXA Two polyethylene sheet products, Aand B, prepared in Thickness, Density, Av. Cell Cell Wall Av. CellHeight Sample Drawn mils g./cc. Width, Thickness, (eale),

microns microns microns A 65 0. 030 300 0. 3 25 1.5x x 1.5 5 0. 400 0. 37 2.0K X 2.0K 2 0. 14 600 0.3 5

a manner similar to Example I exhibit the following properties:

Work-to- A polyethylene sheet product is prepared according to themethod of Example I using 1000 grams of melt index 0.5 polymer, 750 cc.methylene chloride, 20 grams Santocel, and 191 grams ofchlorodifluoromethane. The autogenous pressure of 435 p.s.i. wasincreased to 450 p.s.i. with nitrogen gas, and the product extruded at130 y.p.-rn. through the 0.010" die. The properties were as follows:

The stretch-collapsed Samples B and C have tensile strengths of 18 and24 lb./in.//oz. yd. elongation of 98 and 38%, and moduli of 151 and 157lb./in.//oz./yd. respectively, which may be compared with the much lowerproperties of the uneollapsed as-extruded sheet. In addition, Samples Band C exhibit a much higher surface luster than the uncollapsed (or evena pressure-collapsed) sample of the as-extruded ultramicrocellularsheet.

Microsocpic examination of these bilaterally-stretched examplesindicates that there is a limit to the degree of compliance of theinitially already highly-oriented individual film-like walls of thepolyhedral cells. Thus, some of the cell walls, probably particularlythose initially oriented approximately parallel to the direction ofstretch, will have insufiicient degrees of residual elongation and willbe unable to survive the stretching operation intact. Some of thesewalls separate at their junction with adjacent cell walls while othersundergo various types of failure by tearing throughout their area.However, even Basis Wt. Tensile Work-to-Break Elmendorl Burst VaporTrans Fold Spencer Opacity (on/yd?) (lbs/im/l (in.-lbs./in. Tear(TAPIPI) mission Cycles Puncture (TAPPI),

oz./yd. oz/ydfl) (g.//g./m. (gJmfl/day) (in.-lbs./in. Percent Asextruded 0.5 14.5/26 6.2/1.1 Pressed 0. 5 1S/4.7 13.1/09 Cross lapped(sell bonde 0.9 15.6/163 10.9/11 .3 14.0/90 42 3. 1 250, 000 5. 5 86Cross lapped (adhesive on e 1.2 1l.7/9.7 9.5/0.2 13.1/21 29 3.4 250, 0004.3 86

EXAMPLE X1 in Sample C where the extent of this type of local fail- Thisexample illustrates the production of a collapsed microcellular sheetemploying a bilateral stretching process.

A 34% solution of linear polyethylene of melt index 0.5 was prepared bycharging 74 g./minute of polymer, 96 g./minute of methylene chloride and50 g./minute of chlorodifluoromethane to a heated 2 extruder/rnixer. Thesolution was accumulated in a 10 gallon pressure vessel and subsequentlydischarged at a temperature of 150 C. and superautogenous pressure of550 p.s.i.g. through a 1.75" diameter annular die having a 10 mil gap.The resulting foamed tube was extruded with a small positive internalpressure to inflate the tube and remove the longitudinal folds otherwiseproduced by lateral expansion during the foam generation. The tube wassubsequently slit and opened into a sheet having a tensile strength of13/4 lb./in.//oz./yd. elongation of 179/ 124% and modulus of 18/6lb./in.//o2:./yd. in the machine direction and transverse directionrespectively.

A 4-ply cross-lapped laminate of this sheet was prepared employingCarters Rubber Cement as the adhesive to yield a structure, Sample A,having a thickness of 0.065" and density of 0.030 g./cc. The thicknessof the film-like cell walls is 0.3 micron, and the average transversecell diameter approximately 300 microns. The average cell-height may becomputed from these data to be approximately 25 microns, indicating thatthe method of foam generation plus the slight mechanical pressureemployed in the laminating step has apparently already produced someflattening of the cells.

Two portions of laminated Sample A were heated to 110 C. and stretchedbilaterally simultaneously in two mutually perpendicular directions. Thefirst portion was stretched 1.5x x 1.5x to produce Sample B and thesecond was stretched 2.0x x 2.0x to produce Sample C. The entries in thetable below indicate how strikingly efiicient this treatment is infurther collapsing the cellular ure is more extensive due to the higherdraw ratios employed, the over-all sheet structure is still intact andthe net benefits of tilting the remaining load-bearing elements into theplane of the sheet is obvious from the improved tensile properties. Ofcourse, as the bilateral-s-tretch-ratio is increased beyond 2x, thedegree of internal damage becomes more and more severe, and eventuallyeven the tensile properties of the whole sheet begin to fall ofl.

EXAMPLE XII This example illustrates the increase in modulus ofpressure-collapsed microcellular sheets on annealing.

A linear polyethylene microcellular sheet product was extruded in a.manner similar to Example I by charging 1000 g. polymer containing 5%potassium titanate (in needle form 0.5 micron diameter by 50 micronslong), 750 ml. methylene chloride and 137 g. chlorodifluoromethane intoa 3 liter pressure vessel which was heated, the contents mixed, and thesolution extruded at 150 C. and 500 p.s.i.g. through a 9 mil gap annulardie. Twoply cross-laminated portions of this ultramicrocellular sheetwere compressed at 50 C. in a hydraulic press to densities of 0.14g./cc. (Sample A) and 0.39 g./cc. (Sample B). Three portions of SamplesA and B were then individually mounted between aluminum plates(separated by metal shims to maintain constant sample thickness) andannealed two minutes in steam at pressures of 15 p.s.i.g. (121 C.), 20p.s.i.g. (126 C.) and 25 p.s.i.g. (130 C.). The modulus of the sampleincreases with increasing annealing temperature as follows: Set A, 37(unannealed) 60, 86, 167 lb./in.//oz./yd. and Set B, 74, 190, 210 and300 lb./in.//oz./yd.

EXAMPLE XIII This example illustrates the increase in modulus ofstretch-collapsed microcellular sheets on annealing.

A linear polyethylene microcellular sheet product was extruded in 'amanner similar to Example XI from a 46% polymer solution prepared at 170C. from 106 parts polymer (melt index=0.5), 142 parts methylene chlorideand 1.1 parts Santocel 54. The solution was held for one hour and thenextruded at 150 C. and 700 p.s.i.g. to form an ultrarn'icrocellularsheet product. A 4-ply crosslapped laminate, prepared using CartersRubber Cement, was drawn bilaterally 1.5x x 1.5x at 100 C. to yield astretch-collapsed cellular product. Portions of this sample wereannealed between aluminum plates for two minutes in steam at variouspressures. The modulus of these samples increased from 120 to 183lb./in.//oz./yd. as the steam pressure employed was increased from 16p.s.i.g. (122 C.) to 30 p.s.i.g. (134.5 C.). The rigidity factor (TAPPItest T-45l-M45) increased from 4.8 to 15.

EXAMPLE XIV This example illustrates the production of a microcellularsheet which is collapsed by controlled stretching of the sheet uponextrusion.

Linear polyethylene of melt index 0.5 is fed to a heated 2" Eganextruder provided with a 15/1 L/D screw followed by a 11/1 L/ D mixingsection. Methylene chloride is charged into the molten polymer stream atthe entrance to the mixing section by a McCannimeter pump at a weightflow rate equal to that of the polymer. In addition, 1% by Weight ofSantocel 54 is provided as nucleating agent to assist bubble formationon subsequent extrusion. The 50% solution discharged from the mixingsection is fed into a holding vessel under pressure, and its temperatureis brought to 150 C. When temperature equilibrium is reached, a valve isopened to supply solution at a pressure of 300 p.s.i.g. to a 1.5"diameter annular die of .005" gap width with .010" land length. Theseamless cellular tube which is generated is led between pinch rollsdriven at a surface speed of 125 y.p.m. Diffusion of methylene chloridevapor into the interior of the tube provides an internalsuper-atmospheric pressure which is regulated at approximately 5" watergage pressure by bleeding off excess vapor through a tube, provided witha valve, which leads from the atmosphere to the center of the die. Thisinternal pressure is suflicient to expand the tube to a diameter of6.7". The enforced longitudinal and lateral drawing imposed by thelateral expansion and driven pinch rolls provides a sheet weighing 0.14oz./yd. contrasted with a (pleat-free) free-fall sheet weighing 0.3oz./yd. In addition to reducing the sheet by a factor of 2, the biaxialdrawing produces a fold denser collapsed-cell product of density 0.18g./cc. compared with a density of 0.018 g./cc. for the free-fallproduct.

EXAMPLE XV A linear polyethylene collapsed-cell sheet is extruded in aprocess similar to Example XIV except that the activating liquid isfiuorotriohloromethane, the solution concentration is 50%, 0.75 percentSantocel is used, the extrusion temperature is 145 C., the extrusionpressure is 300 p.s.i.g., the annular extrusion die has a .010" gap anda .020" land, the pinch roll surface speed is 110 y.p.m. and the tube isexpanded laterally by the superatmospheric internal pressure by a factorof 7.2x. This biaxially stretched tubular product has a weight of 0.27oz./yd. and a density of 0.28 g./cc., indicative of its highly collapsedstate as compared with 0.45 oz./yd. and 0.02 g./cc. for the free-fallproduct.

Separate portions of the collapsed sheet are annealed for one minute insteam at temperatures of 124 and 130 C. to produce even higher degreesof internal bonding as indicated by the following properties:

Annealing Treatment (1 minute in steam) None 124" C. C.

Sheet wt. (on/yd?) 0.27 23 .27 Opacity (percent) 60 50 39 Sheet Density(g /cc,) 0. 28 0. 26 0. 32 Tensile St. (lbs. in.//oz./yd.

MD 8. 4 10.0 9.7 8.3 S. 5

For comparison, the initial modulus and force at 5% elongation (ameasure of stiffness) for low density uncollapsed, unbonded, free-fallproducts is approximately 15 lb./in.//oz./yd. and 0.6 lb./in.//oz./yd.

What is claimed is:

1. An integral sheet of a crystalline hydrocarbon polymer comprisingflattened polyhedral shaped cells whose walls have an average filmthickness below 2 microns, possess uniplanar orientation and are alignedsubstantially within the plane of the sheet.

2. The sheet product of claim 1 wherein the smallest dimension of thepolyhedral shaped cells averages less than 50 microns and the secondlargest dimension of the polyhedral shaped cells averages less than 3000microns but at least 3 times the smallest dimension.

3. The product of claim 1 wherein the hydrocarbon polymer ispolyethylene.

4. The product of claim 1 wherein the hydrocarbon poly-mer ispolypropylene.

5. The product of claim 1 wherein the hydrocarbon polymer is selectedfrom the class consisting of linear polyethylene, polypropylene, andcrystallizable copolymers and graft polymers of ethylene and propylenewith l-olefins of up to ten atoms.

6. The sheet product of claim 1 containing acicular potassium titanateuniformly distributed therethrough.

7. The sheet of claim 1 adhesively bonded to fibrous reinforcingelements.

8. A cross-lapped product having at least two bonded plies of cellularsheets whose machine directions are at angles greater than 30 to eachother, the cellular sheets being integral structures of a crystallinehydrocarbon polymer comprising flattened polyhedral shaped cells whosewalls have an average film thickness below 2 microns, possess uniplanarorientation, and are aligned substantially within the plane of thesheet.

9. A process for preparing paper-like and semi-textilelike structureswhich comprises compressing beyond its yield point to a thickness lessthan one-third its original thickness, an integral, crystallineultramicrocellular sheet material composed of a hydrocarbon polymer,substantially all of the polymer in said sheet material being present asfilmy elements of a thickness less than 2 microns with the polymer inthe cell Walls exhibiting uniplanar orientation and a uniform texture.

10. The process of claim 9 wherein the pressing is carried out attemperatures below the crystalline melting point of the polymer.

I 11. The process of claim 9 wherein the hydrocarbon polymer ispolyethylene.

12. The process of claim 9 wherein the hydrocarbon polymer ispolypropylene.

13. The process of claim 9 wherein the step of compressing is followedby subjecting the thickness reduced sheet to bilateral restrainingforces while heating it to a temperature between the glass transitiontemperature and crystalline melting point of the polymer.

14. A process for preparing paper-like and semi-textile-like structureswhich comprises bilaterally stretching, beyond its yield point to athickness less than onethird its original thickness, an integral,crystalline ultramicrocellularsheet material composed of a hydrocarbonpolymer, substantially all of the polymer in said sheet material beingpresent as filmyelements of a thickness less than 2 microns, with thepolymer in the cell walls exhibiting uniplanar orientation and a uniformtexture.

15. The process of claim 14 wherein the bilateral stretching is carriedout at temperatures below the crystalline melting point of the polymer.

16. The process of claim 14 wherein the hydrocarbon polymer ispolyethylene.

17. The process of claim 14 wherein the hydrocarbon polymer ispolypropylene.

18. The process of claim 14 wherein the bilateral stretching is effectedto stretch the sheet material to at least about 150% of its initiallarger dimensions in two mutually perpendicular directions,

19. The process of claim 14 wherein the step of bilateral stretching isfollowed by subjecting the thickness reduced sheet to bilateralrestraining forces while heating it to a temperature between the glasstransition temperature and crystalline melting point of the polymer.

References Cited UNITED STATES PATENTS 2,841,470 7/1958 Berry 25262 X2,877,500 3/1959 Rainer et al 264-22 3,003,304 10/1961 Rasmussen Q'156229 X 3,022,541 2/1962 Passley et al. 264-291 3,050,432 8/1962Weinbrenner et al. 156196 3,072,584 1/1963 Karpovich 26453 X 3,104,1929/1963 Hacklander 156209 FOREIGN PATENTS.

' 911,995 12/ 1962 Great Britain.

1,270,540 7/ 1961 France.

EARL M. BERGERT, Primary Examiner.

20 H. F. EPSTEIN, Assistant Examiner.

